The present invention relates to a fuel injection system for an externally ignited internal combustion engine. The system according to the present invention operates on fuel which is continuously injected into the suction tube of the engine in which a sensing element and an arbitrarily operable butterfly valve are disposed in series. The sensing element is displaced by and in proportion to the quantity of air flowing through the suction tube against a restoring force. In the course of its excursion, the sensing element displaces a movable component of a valve which is disposed in the fuel supply line for metering a quantity of fuel which is proportionate to the quantity of air. The above noted restoring force is supplied by liquid under pressure which is delivered continuously under constant, but arbitrarily variable pressure through a control pressure line. The pressurized liquid exerts a force on a control plunger transferring the restoring force. The pressure of the pressurized liquid is variable by at least one pressure control valve controllable as a function of the engine parameters. The pressure control valve preferably takes the form of a flat seat valve comprising a valve membrane (diaphragm) as the movable valve part.
Fuel injection systems of this type are designed to automatically provide a good fuel - air mixture for all operating conditions of the internal combustion engine so as to burn the fuel as completely as possible and thus prevent toxic gases from being produced, or at least to considerably reduce the same while obtaining maximum performance of the internal combustion engine with minimum fuel consumption. The quantity of fuel must, therefore, be very accurately metered in accordance with the requirements of each operating state of the internal combustion engine.
In the case of known fuel injection systems of this type, the quantity of fuel which is metered is, as far as possible, proportionate to the quantity of air flowing through the suction tube. The ratio of the quantity of fuel which is metered to the quantity of air may be varied by changing the restoring force of the sensing element as a function of the operating parameters by means of a control valve.
It has been found that it is advantageous to momentarily enrich the fuel - air mixture when the butterfly valve of the internal combustion engine is suddenly opened.